This invention relates to a method of producing a paper coating composition which is sufficiently fluid at ambient temperatures for ease of coating but coagulates upon the application of heat to provide a uniform moldable coating which yields a high quality finish for cast coated paper.
Conventional paper coating includes the use of a mineral pigment as the primary component of a paper coating composition to provide a high quality finish for paper products. A typical component of the pigment containing coating includes a proteinaceous binder for adhesion of the mineral pigment to the paper surface. Isolated soy protein has successfully filled this need and is a commonly used adhesive binder for paper coating compositions containing mineral pigments.
There are certain types of high quality coated papers however which are coated by specific coating techniques which require the use of a coating composition that is relatively fluid at ambient temperatures or typical temperatures of coating but which coagulate upon the application of heat to provide a coating with a high quality gloss and uniform finish. While the present invention is not intended to be limited by specific types of paper coating techniques for which the coating composition of the present invention is suitable, nevertheless, the coating containing the binder is particularly well suited for the cast coating of paper. This method which is specifically described in U.S. Pat. No. 1,719,166 generally employs a heated surface against which the rewetted coated paper is pressed until dry. The coagulated coating is softened by the application of boiling water as the coated paper comes into contact with a heated polished drum surface. This resoftening of the coating allows the surface of the coated paper to conform to the polished drum. The heated surface usually conists of a highly polished nickel or chromium roller having a smooth unblemished surface. Drying of the coated paper in this way permits the coating to take on the finish of the drying roll surface, making it possible to obtain a uniformly high gloss finish as long as the surface of the dryer roll remains unmarred.
The usefulness of such a coating process and the quality of coating obtained thereby is greatly improved by the use of a coating which is relatively easy to apply at the temperature of coating but coagulates or thickens upon the application of heat to provide a high quality coating at the moment of heating thereby maximizing the finish and quality of the coating composition.
U.S. Pat. No. 3,356,517 describes a process for the production of a paper coating composition which is operable in a cast coating process by the formation of a coating which is coagulable upon the application of heat. A mineral pigment, a dispersing or suspending agent, a mineral acid or salt thereof, and a proteinaceous adhesive binder provide a coating which heat flocculates at a temperature of 35.degree. to 90.degree. C. It is indicated in this reference that either soy protein or casein may be employed as the adhesive binder for the pigment containing coating.
In spite of the above described disclosure for the production of heat coagulable coatings with either casein or soy protein as an adhesive binder, the use of soy protein binders in such coatings has been extremely inconsistent and not totally reproducible. A need therefore exists for the production of heat coagulable paper coatings containing soy protein as a binder which is consistently reproducible and usable on a commercial basis.
It is therefore an object of the present invention to provide a process for producing a heat coagulable coating composition having a soy protein adhesive binder which consistently coagulates upon the application of heat to provide a uniform coating.
It is a further object to provide a process for producing a heat coagulable paper coating and with a soy protein adhesive binder which is particularly well suited for use in a cast coating process for paper.
These and other objects are accomplished by the present invention as described below.